THE CHLOROPHYLL APPARATUS 171 



proved the same thing by throwing the spectrum of solar 

 light upon a darkened leaf, when he found that starch was 

 produced only in the positions of those same absorption 

 bands, indicating that those were the only places of photo- 

 synthetic activity. 



The process of photosynthesis has been found to pro- 

 ceed under certain circumstances in light which is too 

 feeble in intensity to cause the development of chlorophyll. 

 It is effected in these cases by the etiolin, which we have 

 seen to be the antecedent of chlorophyll. The photo- 

 synthetic power of etiolin is, however, exceedingly small. 



The percentage of carbon dioxide admitted to the 

 chloroplasts has some influence upon the activity of 

 the process. Normal air contains a mere trace of the gas, 

 about 4 parts 'in 10,000. A more copious supply is, how- 

 ever, distinctly advantageous, and the activity increases as 

 the percentage rises. The optimum quantity appears to be 

 about 10 per cent, with light of the ordinary intensity. 

 More than this gradually exerts a paralysing influence on 

 the chloroplasts, and sets up consequently an inhibition of 

 the apparatus. A similar inhibition can be caused by the 

 accumulation of the products of its activity, a concentration 

 of the sugar amounting to 8 per cent, being sufficient to 

 bring it about. 



The mechanism is an exceedingly delicate one and can 

 be thrown out of gear by various external agencies. 

 Ewart has shown that it can be inhibited by heat, cold, 

 desiccation, partial asphyxiation, prolonged insolation, and 

 by the action of dilute alkalis or mineral acids. 



We mentioned at the commencement of this chapter 

 that the chlorophyll apparatus is concerned in the manu- 

 facture of almost the whole of the organic material of the 

 globe. In a few humble organisms the construction of 

 such material can proceed without its help. These are 

 certain bacteria which can transform ammonia compounds 

 into salts of nitrous and nitric acids, growing and multi- 

 plying at the expense of the products they thus obtain 



